Some online search and navigation systems use attribute-value pairs to characterize a collection of items. Such systems allow a user to select values for one or more attributes, and return the subset of the items characterized by the selected values. In order to permit the user to further refine his search, these systems present additional values that characterize at least a portion of the returned subset. This iterative process allows the user to arrive at a desired subset of items via multiple search and refinement paths.
The following references, which are incorporated herein by reference, may be of interest:    G. Kahn and G. Plotkin, “Concrete domains,” Theoretical Computer Science 121:187-277 (1993)    G. Berry and Pierre-Louis Curien, “Sequential algorithms on concrete data structures,” Theoretical Computer Science 20:265-321 (1982)    Glynn Winskel, “Event structures,” Advances in Petri nets 1986, Springer Lecture Notes in Computer Science 255 (1987)